This invention relates to the illumination arts. More particularly, this invention relates to a light emitting system incorporating a plurality of light emitting diodes (LEDs) or laser diodes (LDs) and phosphor materials, which is capable of producing visible white or colored light having a desired light distribution pattern.
A color-mixing lighting system is known from U.S. Pat. No. 6,234,648. The known color-mixing lighting system comprises at least two light emitting diodes each emitting, in operation, visible light in a pre-selected wavelength range. A converter converts part of the visible light emitted by one of the LEDs into visible light in a further wavelength range so as to optimize the color rendition of the lighting system. Preferably, the diodes include a blue light emitting diode and a red light emitting diode and the converter includes a luminescent material for converting a portion of the light emitted by the blue light emitting diode into green light.
It is also known to combine blue, green and red light emitting diodes (LEDs) in a color mixing system to make white light for general lighting applications. The correlated color temperature (CCT) can be set by properly tuning the power ratio of the individual LEDs. If the spectral emission band wavelength of the three LEDs is in the ranges 430-470 nm, 520-560 nm, and 590-630 nm, a color rendering index (CRI) of about 80-85 is possible. In addition, it is known that the emission spectrum of a LED typically exhibits a single, relatively narrow peak at a wavelength (“peak wavelength”) determined by the structure of the light emitting diode and the composition of the materials from which the LED is constructed. This implies that combining blue, green and red LEDs to form a light source of white light limits the achievable CRI. In addition, the obtainable color rendering index is very sensitive to small wavelength variations of the LEDs.
Light emitting diodes and lasers have been produced from Group III-V alloys, such as gallium nitride (GaN). To form the LEDs, layers of GaN-based alloys are typically deposited epitaxially on a substrate, such as silicon carbide or sapphire, and may be doped with a variety of n and p-type dopants to improve properties, such as light emission efficiency. Such GaN-based LEDs generally emit light in the UV and/or blue range of the electromagnetic spectrum.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Performance improvements have enabled new applications for LEDs of saturated colors in traffic lights, exit signs, store signs, and the like.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs include blue emitting InGaN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complementary color, e.g. yellow. Together, the blue and yellowish radiation produces a white light. There are also white LEDs that utilize a near UV emitting chip and a phosphor blend including red, green and blue-emitting phosphors designed to convert the UV radiation to visible light.
Known white light emitting devices comprise a blue light emitting LED having a peak emission wavelength in the blue range (from about 430 nm to about 480 nm) combined with a yellow light emitting phosphor, such as cerium (III) doped yttrium aluminum garnet (“YAG:Ce”), a cerium (III) doped terbium aluminum garnet (“TAG:Ce”), or a europium (II) doped barium orthosilicate (“BOS”). The phosphor absorbs a portion of the radiation emitted from the LED and converts the absorbed radiation to a yellow light. The remainder of the blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. A viewer perceives the mixture of blue and yellow light as a white light. The total of the light from the phosphor material and the LED chip provides a color point with corresponding color coordinates (x and y) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
The wavelength of the light emitted by the phosphor is dependent on the particular phosphor material used. For example, a blue absorbing, yellow emitting phosphor, such as YAG, can be used to generate yellow light. Light sources produced in this manner are suited to a wide variety of applications, including lamps, displays, back light sources, traffic signals, illuminating switches, and the like.
Other white light LED sources use different colored LED chips rather than phosphor converted LEDs. Lighting systems which use LEDs to produce white light are more efficient at the package level than lighting systems which use phosphor-LEDs. However, high quality white light is more difficult to achieve in solely LED based lighting systems. This is because LEDs manufactured to optimize total lighting system performance and production typically must be combined in an undesirably large integral number of LED chips to provide the requisite quantities of red, green and blue light when operated at full rated power. Moreover, using LEDs having a wide variety of different hues would necessitate using LEDs having a variety of different efficiencies, thereby reducing the efficiency of the system.
There are other limitations associated with solely LED based lighting systems. Existing green LEDs operating at the very desirable light spectral wavelength of about 550 nm are very inefficient. Further, currently available efficient LEDs make good color rendering difficult to achieve. Good color rendering is possible, but places constraints on specific choices of LEDs.
Additionally, mixing LEDs to produce white light adds efficiency costs. More specifically, many highly collimated mixing schemes are binary in that they mix two LEDs at a time. Solely, LED based lighting systems typically use three and four LEDs and thus, require two stages of mixing. Unfortunately, each stage of mixing has an efficiency cost which significantly lowers the performance of the system.
Thus, as alluded to earlier, there are advantages to producing white light with phosphor-LED based lighting systems as compared to solely LED based lighting systems because phosphor-LEDs do not require mixing and have lower material costs (they are inherently mixed). However, they tend to be less efficient at the package level than LED based lighting systems because of quantum deficits and cross-excitation losses.
Accordingly, there is a need for a lighting system which combines certain aspects of LED and phosphor-LED based lighting systems to achieve benefits beyond either system, including high luminous efficacy, high CRI over a wide range of CCT values, and a better color control to achieve and maintain the color point.